1. Field of the Invention
The present invention relates to high speed tool steel produced by sintering powder for use in a cutting tool or a cold heading tool and exhibiting both excellent wear resistance and satisfactory toughness under a high speed operational condition in which hardness and wear resistance are required at high temperature and a method of producing the same.
2. Related Art
High speed tool steel for use in a cutting tool or a cold heading tool must exhibit superior wear resistance with high hardness and excellent toughness.
There have been disclosed a variety of methods of improving the toughness of high speed tool steel produced by melting; for example, there has been disclosed a method in which Nb and other elements are added to make crystal grains fine in size to improve toughness (as shown in Japanese Patent Laid-Open No. 58-73753 and Japanese Patent Laid-Open No. 58-117863). Another method has been disclosed in which Nb and rare earth element are added to provide MC-type carbides finely distributed uniformly which carbides are mainly composed of Nb, to thereby improve toughness (as disclosed in Japanese Patent Publication No. 61-896).
On the other hand, regarding the improvement of wear resistance, in a case of high speed tool steel produced by sintering powder, in which steel it is possible to uniformly distribute fine carbide grains and to make the crystal grains fine in size, it has been most usual to increase the amount of carbides. For example, in Japanese Patent Publication Nos. 57-2142 and 55-148747, W equivalent mainly made to be in a high level to thereby increase the amount of M.sub.6 C-type carbides mainly composed of W and/or Mo, so that wear resistance is improved because of increased hardness.
Furthermore, in a high speed tool steel produced by sintering powder, it is proposed to add Nb for the purposes of making crystal grains fine in size and preventing grains from becoming coarse in size even when austenitizing temperature is high level as shown in Metall. Trans. 19A (1988) p. 1395 to 1401 and Japanese Patent Laid-Open No. 1-212736).
However, in the high speed tool steel produced by melting in Japanese Patent Laid-Open Nos. 58-73753 and 58-117863, the excessive addition of Nb causes the occurrence of crystallized coarse carbides of NbC essentially composed of Nb. Also coarse carbides are, at the time of the solidification, crystallized which are M.sub.6 C-type carbides essentially composed of W and Mo. Therefore, the effect of improving toughness by making crystal grains fine is diminished, with the result that the toughness is undesirably deteriorated.
Furthermore, although in the high speed tool steel produced by sintering powder it has been effected to increase the quantity of carbides or to make the hardness of the tool high for improving wear resistance, toughness is undesirably deteriorated, causing a problem of a breakage or cracking of the tool.
In the high speed tool steel of the Japanese Patent Laid-Open No. 55-148747 produced by sintering powder to which Nb is added, Nb is only intended to form hard carbide by adding Nb in place of V.
In the high speed tool steel disclosed in Metall. Trans. 19A (1988) p. 1395 to 1401 and Japanese Patent Laid-Open No. 1-212736, the addition of Nb makes it possible to enhance the quenching temperature while preventing the coarsening of crystal grains. However, the inventors of the present invention have found that in that steel there is insufficient resistance to softening upon high temperature-tempering, such resistance is required at the high temperatures encountered in a severe use thereof. This is due to the low content of alloying elements, in particular, due to low level of W equivalent, and wear resistance is also insufficient due to the small amount of carbides.
Therefore, the above-shown conventional high speed tool steel cannot satisfy the tool usage condition required in recent years in which a higher speed operation is needed.